Itsuki Koizumi is an esper from "The Agency" that acquired the ability to explore "Closed Space" and combat Celestials three years prior to the "explosion of data" witnessed by others. He's a person found to be smiling all the time, and is to be carefree. Itsuki Koizumi's Agency is rivaled by Yuki Nagato's and Mikuru Asahina's organizations are not really friendly with each other. Itsuki Koizumi confuses Kyon with Yuki Nagato's and Mikuru Asahina's side of their story. Kyon seems to always be annoyed by him.